1. Technical Field
The invention relates to exhaust gas recirculating systems for internal combustion engines and more particularly to an exhaust gas recirculating system for a turbocharged diesel engine.
2. Description of the Problem
Turbocharging is a well known method for increasing the efficiency and boosting the peak power output of an internal combustion engine. An exhaust energy recovery turbine is positioned in the exhaust stream from the engine and uses energy from the exhaust gas to drive a supercharger disposed in the engine's air intake system. The supercharger boosts the density of air delivered to the engine's intake manifold. The increased air density allows additional fuel to be introduced to the cylinders and combusted, increasing engine output. Turbocharging, while used occasionally with spark ignition engines, is more commonly found in compression ignition engines, i.e. diesel engines.
Complicating the application of turbocharging to diesel engines is the need to meet government emission standards, particularly those relating to NOx emissions. One way in which emissions are reduced is through the use of exhaust gas recirculating systems (EGR) in which exhaust gas is returned to the air intake system. The air intake system for a supercharged diesel engine includes the compressor pump and, usually, an intercooler between the supercharger and the engine's intake manifold. Exhaust gas can be returned ahead of the supercharger, into the intercooler, or directly into the intake manifold. Exhaust gas may be drawn from the exhaust manifold ahead of the energy recovery turbine, or from some point further down the exhaust system. The most common arrangement is to draw exhaust gas directly from the exhaust manifold and to deliver the gas to intake manifold, usually passing the exhaust gas through an intercooler in the EGR line. See, by way of example, U.S. Pat. No. 6,470,864 to Kim et al., U.S. Pat. No. 6,247,311 to Itoyama et al., and U.S. Pat. No. 6,412,278 to Matthews. It is usually considered undesirable to return the exhaust gas to the intake system ahead of the supercharger or the intake intercooler due to the potential of corrosion the exhaust gas poses for these components. The exhaust gas is considerably hotter than ambient air and this reduces the flow mass through the supercharger compressor and reduces the power output of the engine. While Kim et al. and Itoyama et al. provided no pressure boost for the exhaust gas, Matthews stated that the exhaust gas from the exhaust manifold may not have sufficient pressure to allow smooth delivery of the gas into the intake manifold due to pressures in the intake manifold. Matthews provided a hydraulically driven pump in the exhaust gas recirculating line to assure sufficient exhaust gas pressure for delivery to the intake manifold.
Many prior art EGR systems have the disadvantages of introducing an extremely high temperature gas to the engine induction system and of diverting exhaust air flow from the exhaust turbine, reducing the turbine's effectiveness. While all position an intercooler in the EGR line to reduce exhaust gas temperature, these systems still release a great deal of heat into a vehicle's engine compartment, promoting engine overheating. In addition, the need for an intercooler in the EGR line adds substantial expense to the systems.
Cook, U.S. Pat. No. 6,470,866 proposed diverting exhaust gas from the exhaust pipe downstream from the exhaust turbine. This was achieved by restricting flow through the exhaust pipe to boost the pressure of the gas. However, this arrangement increases exhaust system back pressure which again reduces exhaust turbine performance. The prior art proposals considered have all had the disadvantage of reducing peak engine output.